The optically active liquid crystal compounds of the present invention are antiferroelectric liquid crystals having a ferroelectric property showing bistable states and also having an antiferroelectric property showing really novel optical tristable states. The liquid crystal compounds as well as liquid crystal compositions containing the compound can be used for display elements as well as electrooptical devices utilizing responses of the liquid crystal compound to electric field.
As applications of liquid crystals, electrooptical apparatuses using a nematic liquid crystal such as a DSM cell, TN cell, G-H cell, or STN cell have been developed and practically used. However, all of the electrooptical apparatuses using such a nematic liquid crystal have a defect that response time is as slow as several msec to several tens msec, leading to a considerable restriction in their applications. The slow response of the electrooptical apparatuses or elements using the nematic liquid crystal is due to the fact that the torque which changes the position of molecules is inherently based on the anisotropy of dielectric constant, and thus its force is not so strong. With such a technical background, the development of ferroelectric liquid crystals had been attempted which have a spontaneous polarization (Ps), have a strong torque based on Ps.times.E (E is an applied voltage), and have an extremely short optical response time of few .mu.sec to several tens of .mu.sec to make the preparation of hypervelocity devices possible.
Mayer et al. synthesized DOBAMBC (p-decyloxy-benzilidene-p-ammino-2-methylbutyl cinnamate) in 1975 for the first time in the world and it has been confirmed to be a ferroelectric liquid crystal (Le Journal de Physique, Vol. 36, 1975, L-69).
Further, since Clark and Lagawall reported in 1980 on such characteristics on display devices as high velocity response of submicroseconds and memory characteristics of DOBAMBC, ferroelectric liquid crystals have absorbed considerable public attention (N. A. Clark et al., Appl. Phys. Lett. 36, 899 (1980)).
However, there were many technical problems in their system for practical use. Particularly, there was not any available material showing ferroelectric liquid crystallinity at an ambient temperature, nor was effective and practical method established for controlling the alignment of liquid crystal molecules control of which is essential for display devices.
After the publication of the report, various attempts have been made from both aspects of liquid crystal material and device. Display devices utilizing the switching between twisted bistable states were prepared for trial, and high speed electrooptical apparatuses using the device are proposed in U.S. Pat. No. 4,367,924 and others. However, high contrast and proper potential of threshold value have not been obtained.
From such a point of sight, other switching systems were explored to propose a transitional diffusion system. Subsequently, a three states switching system of a liquid crystal having tristable states was reported in 1988 (A. D. L. Chandani, T. Hagiwara, Y. Suzuki et al., Japan, J. of Appl. Phys., 27, (5), L729-L732 (1988)).
The optical tristable states herein referred to mean that, when a voltage in the form of a triangular wave as shown in FIG. 1 A is applied to a liquid crystal electrooptical device in which an antiferroelectric liquid crystal is laid between the first electrode substrate plate and the second electrode substrate plate which are apart at a given space from each other, the antiferroelectric liquid crystal shows the first stable molecular orientation, that is, the first optically stable state which corresponds to the point 2 in FIG. 1 D when electric voltage is zero. The antiferroelectric liquid crystal shows the second stable molecular orientation, that is, the second optically stable state which corresponds to the point 1 in FIG. 1 D in the electric field of one direction, and shows the third stable molecular orientation, that is, the third optically stable state which corresponds to the point 3 in FIG. 1 D in the electric field of other direction.
Liquid crystal electrooptical apparatuses utilizing the tristable states, that is, three states are proposed in U.S. Pat. No. 5,046,823 filed by the present applicant.
The characteristics of an antiferroelectric liquid crystal showing the tristable states are described in more detail below.
In the ferroelectric liquid crystal element having a stabilized surface which was proposed by Clark-Lagawall, ferroelectric liquid crystal molecules show two stable states in which the molecules are uniformly oriented or aligned in one direction in the phase S*C. The molecules are stabilized in either state depending on the direction of applied electric field as shown in FIGS. 2 (a) and (b), and the states are kept even when the electric field was shut off.
Actually, however, the alignment of the ferroelectric liquid crystal molecules shows twisted two states in which directors of the liquid crystal molecules are twisted or the molecules show a chevron structure in which layers of the molecules are bent in a doglegged shape. In the chevron layer structure, a switching angle becomes small, forming a cause for a low contrast, which constitutes a serious obstacle for its practical use.
On the other hand, in the liquid crystal electrooptical devices, an "anti" ferroelectric liquid crystal molecules are aligned in antiparallel, tilting in opposite direction at every adjoining layer, in the phase S*.sub.(3) showing the tristable states, and thus the dipoles of the liquid crystal molecules are negating each other. Accordingly, the spontaneous polarization is nullified as a whole. The transmittance of the liquid crystal phase showing such molecular alignment corresponds to the point 2 in FIG. 1 D.
Further, when a voltage sufficiently higher than a threshold value of (+) or (-) was applied, liquid crystal molecules are tilted in the same direction and aligned in parallel as shown in FIGS. 3 (b) and (c). In these states, the spontaneous polarization is produced since the dipoles are also shifted to the same direction to form a ferroelectric phase, and the transmittance of the liquid crystal phase in those states correspond to the points 1 or 3 in FIG. 1 D.
That is, in the phase S*.sub.(3) of the "anti" ferroelectric liquid crystal, the "anti" ferroelectric phase at the time of no-electric field and also two ferroelectric phases due to the polarity of applied electric field are stabilized, and switching is carried out among the tristable states of an "anti" ferroelectric phase and two ferroelectric phases, with a direct current-like threshold value. Based on the change in the alignment of liquid crystal molecules accompanied with the switching, light transmittance is changed while drawing such a double hysteresis as shown in FIG. 4.
One of the characteristics of the present invention is that a memory effect can be realized by applying a bias voltage and then further applying a pulse voltage to the double hysteresis as shown in FIG. 4 (A).
Moreover, the ferroelectric phase is stretched in terms of its layer by the application of an electric field to form a book-shelf structure. On the other hand, in the "anti" ferroelectric phase of the third stable state, an analogous book-shelf structure is formed. Since the switching of the layer structure due to the application of an electric field gives a dynamic shear to liquid crystal layers, alignment defects are improved during driving, and thus a good molecular alignment can be realized.
In the "anti" ferroelectric liquid crystal, since image display is performed by alternatively using both hysteresises of plus side and minus side, after-image phenomenon due to the accumulation of inner electric field based on the spontaneous polarization can be prevented.
As explained above, the "anti" ferroelectric liquid crystal compound can be said to be very useful since it has advantages as follows:
1) Hipervelocity response is possible, PA1 2) High contrast and wide viewing can be expected, and PA1 3) Excellent alignment characteristics and a memory effect can be realized. PA1 1) The material exhibits an "anti" ferroelectric phase S*CA at a broad temperature range including an ambient temperature, PA1 2) The material shows a high speed response in an order of ten odd .mu.sec at a temperature range practically used, PA1 3) The material has a high threshold value suitable for display driving, and PA1 4) The material has a stable and good alignment characteristic.
Reports have been made on the liquid crystal phase of the "anti" ferroelectric liquid crystal showing the tristable states in the following articles:
1) A. D. L. Chandani et al., Japan J. Appl. Phys., 28, L-1265 (1989), and
2) H. Orihara et al., Japan J. Appl. Phys., 29, L-333 (1990).
The liquid phase is called "Phase S*CA" (Antiferro-electric Smectic C* phase) in association with the "anti" ferroelectric property. The phase is named "phase S*.sub.(3) " in the present specification since the liquid crystal phase performs the switching among tristable states.
The liquid crystal compounds which have the "anti" ferroelectric phase S*.sub.(3) showing the tristable states in a phase series are disclosed in Japanese Unexamined Patent Publication No. 1-316367, U.S. Pat. Nos. 5,171,471 and 4,973,738, and European Patent No. 330,491 A filed by the present inventors, and in Japanese Unexamined Patent Publication No. 1-213390 filed by Ichihashi et al. Liquid crystal electrooptical devices utilizing the tristable states are proposed in Japanese Unexamined Patent Publication No. 2-40625 and U.S. Pat. No. 5,046,823 filed by the present inventors.
The liquid crystal compounds having an amide linkage are reported in Japanese Unexamined Patent Publication Nos. 63-126865, 63-132869, and 2-151684.
However, Japanese Unexamined Patent Publication No. 63-126865 has disclosed optically active, cyclic amide compounds such as indole ring compounds, and Japanese Unexamined Patent Publication No. 63-132869 has disclosed compounds prepared by using L-isoleucine derived from a natural substance as a starting raw material. Both of them are chiral dopant compounds which produce a ferroelectric chiral smectic phase when blended in an amount of about 5% to a base liquid crystal.
Further, Japanese Unexamined Patent Publication No. 2-151684 has proposed the use of amides such as dimethyl formamide, dibutyl formamide, and diphenyl formamide as a stabilizer to a change with the passage of time of a liquid crystal phase.
As will be understood from the above, any report has not yet been published on an "anti" ferroelectric liquid crystal having an amide linkage.
When liquid crystal materials to be actually used are produced at the present time, it is seldom or never that a liquid crystal compound is used by itself. Usually, few kinds or more of liquid crystal compounds are used in combination as a composition, and if circumstances require, the liquid crystal compound is used together with a not-liquid crystal substance as dopant. This is a reflection of the fact that a single liquid crystal compound has not been developed which satisfy the requirements on practically usable temperature range and electrooptical characteristics.
The "anti" ferroelectric liquid crystal compounds have also the same problem and thus two kinds or more of "anti" ferroelectric liquid crystal compounds are usually mixed to exhibit several characteristics as a composition satisfying the purpose of using the compound.
The following characteristics of materials are required for "anti" ferroelectric liquid crystals:
Among the characteristics of materials, storage temperature range in particular is a basic physical property which is important for developing displays, and the broad temperature range from about -30.degree. C. to about 100.degree. C. is generally required. When a liquid crystal composition having the objective, practical storage temperature range is prepared by mixing few kinds or more of liquid crystal compounds at an appropriate blend ratio to achieve the purpose mentioned above, it becomes necessary to use at least one liquid crystal compound as a component to be blended which exhibits an objective liquid crystal phase at a high temperature range by itself in order to expand the phase transition temperature at a high temperature range. Accordingly, the development or exploration of antiferroelectric liquid crystal compound becomes principally an important subject for putting liquid crystal displays to practical use.
As a result of exploration of various liquid crystal compounds from the view point of putting antiferroelectric liquid crystals to practical use, the liquid crystal compounds have been found which have an amide linkage and show a phase S*CA in a high temperature range, and thus can efficiently be used for expanding the high temperature range of "anti" ferroelectric liquid crystal composition.